1. Field of the Invention
The present invention relates to a backlight inverter for a thin film transistor-liquid crystal display (TFT-LCD) panel, and more particularly to a backlight inverter for an LCD panel of an asynchronous pulse width modulation (PWM) driving type, which is capable of delaying a plurality of pairs of PWM drive signals, which are inputted respectively to power switches to drive a plurality of cold cathode fluorescent lamps (CCFLs) in pairs, sequentially by a predetermined time interval in such a manner that the PWM drive signal pairs corresponding respectively to the lamp pairs have different phases and the lamps thus have different PWM on/off periods, so that overshoot of a power supply circuit can be reduced so as to keep the entire system power stable and so that switching noise based on PWM dimming can be reduced so as to reduce screen noise and increase system reliability.
2. Description of the Related Art
Generally, CCFLs are operated at low current, resulting in advantages such as low power consumption, low heat, high brightness and long life. In this regard, the CCFLs have recently been used in various display devices such as a backlight unit of a computer monitor, for example, a TFT-LCD, and a display panel of a printer. A high alternating current (AC) voltage of about 1-2 kV/several tens kHz is required to light such a CCFL, and an inverter is utilized to provide such a high AC voltage by performing a DC/AC conversion operation with respect to a direct current (DC) voltage of about 5 to 30V.
In such an inverter, each CCFL is turned on with an AC voltage of several tens kHz provided through a power switch, a converter and a transformer oscillator. In the case of being applied to a backlight unit of a computer monitor, CCFLs, typically on the order of 4 to 8, are installed, and controlled with PWM drive signals, respectively.
FIG. 1 is a circuit diagram showing the construction of a conventional backlight inverter for an LCD panel.
With reference to FIG. 1, the conventional backlight inverter comprises power switches SWA and SWB for converting a DC voltage Vcc into square-wave voltages, respectively. The square-wave voltages from the power switches SWA and SWB are boosted and oscillated by converters 120A and 120B, each of which consists of an inductor and a diode, and transformer oscillators 130A and 130B, respectively, such that they are converted into AC voltages of about 1-2 kV/40 kHz for lighting CCFLs 140A and 140B.
At this time, voltages resulting from currents flowing through the lamps 140A and 140B are detected by lamp voltage detectors 150A and 150B, respectively, and then fed to a driving integrated circuit (IC) 110. The driving IC 110 provides PWM drive signals to the power switches SWA and SWB on the basis of the detected lamp voltages, a dimming voltage Vdim and a PWM oscillation signal PWM OSC. Notably, the conventional backlight inverter for the LCD panel employs a PWM dimming system to adjust the brightness of the CCFLs on the basis of the dimming voltage Vdim and PWM oscillation signal PWM OSC.
In the conventional backlight inverter for the LCD panel, PWM drive signals inputted respectively to power switches for the dimming of multiple CCFLs have the same on/off times as shown in FIG. 3.
That is, in the conventional backlight inverter for the LCD panel, the power switches SWA and SWB have their on/off periods synchronized to supply powers to the CCFLs, respectively, in response to a PWM pulse generated according to voltage levels of the PWM oscillation signal and dimming voltage Vdim, so as to adjust the brightness of the CCFLs.
FIG. 2 is a block diagram showing the construction of a conventional PWM driving circuit for driving four lamps.
In the case of driving four lamps using the conventional backlight inverter for the LCD panel as shown in FIG. 1, first and second driving ICs 110A and 110B, each of which is the same as the driving IC 110 shown in FIG. 1, are connected in parallel to drive the four lamps, as shown in FIG. 2.
FIG. 3 is a timing diagram of PWM drive signals for driving the four lamps in FIG. 2.
The PWM drive signals PWM1-PWM4 for respective lamp operations, determined depending on the dimming voltage Vdim and PWM oscillation signal PWM OSC, are synchronized to have the same on times and the same off times. As a results all of the power switches SWA-SWD, operated in response to the PWM drive signals PWM1-PWM4, also have their PWM on/off periods synchronized.
With reference to FIGS. 2 and 3, in the case where the backlight inverter for the LCD panel as shown in FIG. 1 is applied to, for example, four lamps, a PWM pulse is outputted from an NCO (Next Chain Out) terminal of the first driving IC 110A shown in FIG. 2 and then inputted to an NCI (Next Chain Input) terminal of the second driving IC 110B. At this time, the power switches Q1 and Q2 are turned on/off in response to the PWM pulse in the same phases thereof, so current waveforms of the lamps have the same phases as shown in FIG. 3.
However, the aforementioned conventional backlight inverter for the LCD panel has a disadvantage in that a plurality of used lamps are synchronized to have the same PWM on/off periods, resulting in the occurrence of overshoot in a power supply circuit in proportion to the number of the used lamps and the concurrence of noises in power switches, causing an increase in switching noise. Further, an oscillation mode, such as lamp flickering, may take place, resulting in screen blinking.